The present invention relates to a combustor capable of burning fuel in compressed air. More specifically, the present invention relates to a combustor for a gas turbine having venturi fuel-air mixing apparatus and flame stabilization apparatus such that the combustor operates at ultra-low NOx conditions with a stable flame in the combustion zone.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range in firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. High temperatures in the primary combustion zone promote the formation of NOx, considered an atmospheric pollutant.
It is known that NOx formation can be reduced by combustion at lean fuel/air ratios. Such lean burning, however, requires that the fuel be well distributed throughout the combustion air without creating any locally rich zones. Unfortunately, in known combustor arrangements designed to provide lean fuel/air mixtures, air is delivered to the flame-front at the time of combustion at an undesirably low velocity. Burning of the fuel at a low flame velocity is known to cause the flame to travel back upstream ahead of the desired location of the flame-front. Accordingly, fuel at locations upstream of the desired flame-front becomes ignited in low velocity air in a flash-back condition. Flash-back conditions within the combustor are known to increase the formation of NOx beyond acceptable levels, reduce the overall turbine operating efficiency to undesirable levels and result in prolonged destruction of the combustor apparatus.
It is therefore desirable to provide a combustor that is capable of burning fuel at very lean mixtures of fuel and air and wherein the fuel is burned in air at a velocity that is sufficiently high to avoid flashback, so as to reduce the formation of NOx and provide for stable combustion.